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JWJ > Volume 33(5); 2015 > Article
Journal of Welding and Joining 2015 October;33(5) :53-60.
Published online October 30, 2015.
doi:https://doi.org/10.5781/JWJ.2015.33.5.53
Microstructure and Hardness of Yb:YAG Disc Laser Surface Overlap Melted Cold Die Steel, STD11
Kwang-Hyeon Lee*, Seong-Won Choi**, Jung Gil Yun**, Myeong-Hwan Oh**, Byung Min Kim***, Chung-Yun Kang**
*Korea Institute of Machinery & Materials, Busan Laser Application Support Center, Busan 46742, Korea
**Dept. of Material Science and Engineering, Pusan National University, Busan 46214, Korea
***School of Mechanical Engineering, Pusan National University, Busan 46214, Korea
Correspondence: Chung-Yun Kang  Email: kangcy@pusan.ac.kr
  Published online October 30, 2015
ABSTRACT
Laser surface Melting Process is getting hardening layer that has enough depth of hardening layer as well as no defects by melting surface of substrate. This study used CW(Continuous Wave) Yb:YAG and STD11. Laser beam speed, power and beam interval are fixed at 70mm/sec, 2.8kW and 800um respectively. Hardness in the weld zone are equal to 400Hv regardless of melting zone, remelting zone overlapped by next beam and HAZ. Similarly, microstructures in all weld zone consist of dendrite structure that arm spacing is 3~4μm, matrix is γ(Austenite) and dendrite boundary consists of γ and M7C3 of eutectic phase. This microstructure crystallizes from liquid to γ of primary crystal and residual liquid forms γ and M7C3 of eutectic phase by eutectic reaction at 1266℃. After solidification is complete, primary crystal and eutectic phase remain at room temperature without phase transformation by quenching. On the other hand, microstructures of substrate consist of ferrite, fine M23C6 and coarse M7C3 that have 210Hv. Microstructures in the HAZ consist of fine M23C6 and coarse M7C3 like substrate. But, M23C6 increases and matrix was changed from ferrite to bainite that has hardness above 400Hv. Partial Melted Zone is formed between melting zone and HAZ. Partial Melted Zone near the melting zone consists of γ, M7C3 and martensite and Partial Melted Zone near the HAZ consists of eutectic phase around γ and M7C3. Hardness is maximum 557Hv in the partial melted zone.
Keywords: Laser Hardening | Yb:YAG Disc Laser | STD11 | Laser surface melting | Trimming Die
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